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‎README.md‎

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A collection of design patterns and idioms in Python.
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Remember that each pattern has its own trade-offs. And you need to pay attention more to why you're choosing a certain pattern than to how to implement it.
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## Creational Patterns
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> Patterns that deal with **object creation** — abstracting and controlling how instances are made.
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```
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| Pattern | Description |
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|:------- |:----------- |
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| [abstract_factory](patterns/creational/abstract_factory.py) | use a generic interface to create a family of related objects |
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|:-------:| ----------- |
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| [abstract_factory](patterns/creational/abstract_factory.py) | use a generic function with specific factories |
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| [borg](patterns/creational/borg.py) | a singleton with shared-state among instances |
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| [builder](patterns/creational/builder.py) | instead of using complex constructors, isolate the construction of an object and make it multi-step |
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| [factory](patterns/creational/factory.py) | delegate the creation of objects to specialized methods or classes |
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| [lazy_evaluation](patterns/creational/lazy_evaluation.py) | wait until the value is needed to calculate it |
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| [pool](patterns/creational/pool.py) | preinstantiate and maintain a group of objects of the same type |
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| [prototype](patterns/creational/prototype.py) | use a factory to create new objects by copying an existing instance |
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| [builder](patterns/creational/builder.py) | instead of using multiple constructors, builder object receives parameters and returns constructed objects |
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| [factory](patterns/creational/factory.py) | delegate a specialized function/method to create instances |
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| [lazy_evaluation](patterns/creational/lazy_evaluation.py) | lazily-evaluated property pattern in Python |
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| [pool](patterns/creational/pool.py) | preinstantiate and maintain a group of instances of the same type |
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| [prototype](patterns/creational/prototype.py) | use a factory and clones of a prototype for new instances (if instantiation is expensive) |
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| [singleton](patterns/creational/singleton.py) | restrict the instantiation of a class to one object |
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## Structural Patterns
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| Pattern | Description |
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|:------- |:----------- |
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|:-------:| ----------- |
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| [3-tier](patterns/structural/3-tier.py) | data<->business logic<->presentation separation (strict relationships) |
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| [adapter](patterns/structural/adapter.py) | adapt one interface to another using a white-box wrapper |
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| [bridge](patterns/structural/bridge.py) | decouple an abstraction from its implementation |
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| [composite](patterns/structural/composite.py) | encapsulate a group of objects into a single object |
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| [decorator](patterns/structural/decorator.py) | wrap a class to add new functionality without changing its structure |
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| [facade](patterns/structural/facade.py) | provide a simplified interface to a complex system |
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| [flyweight](patterns/structural/flyweight.py) | use sharing to support a large number of objects efficiently |
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| [front_controller](patterns/structural/front_controller.py) | a single entry point for all requests to an application |
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| [proxy](patterns/structural/proxy.py) | an object representing another object |
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| [mvc](patterns/structural/mvc.py) | separate data (model), user interface (view), and logic (controller) |
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| [adapter](patterns/structural/adapter.py) | adapt one interface to another using a white-list |
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| [bridge](patterns/structural/bridge.py) | a client-provider middleman to soften interface changes |
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| [composite](patterns/structural/composite.py) | lets clients treat individual objects and compositions uniformly |
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| [decorator](patterns/structural/decorator.py) | wrap functionality with other functionality in order to affect outputs |
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| [facade](patterns/structural/facade.py) | use one class as an API to a number of others |
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| [flyweight](patterns/structural/flyweight.py) | transparently reuse existing instances of objects with similar/identical state |
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| [front_controller](patterns/structural/front_controller.py) | single handler requests coming to the application |
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| [mvc](patterns/structural/mvc.py) | model<->view<->controller (non-strict relationships) |
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| [proxy](patterns/structural/proxy.py) | an object funnels operations to something else |
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## Behavioral Patterns
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| Pattern | Description |
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|:------- |:----------- |
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| [chain_of_responsibility](patterns/behavioral/chain_of_responsibility.py) | allow multiple objects to handle a request without them needing to know about each other |
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| [command](patterns/behavioral/command.py) | encapsulate a request as an object, allowing for parameterization and queuing |
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| [catalog](patterns/behavioral/catalog.py) | a class that allows looking up other classes based on various criteria |
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| [chaining_method](patterns/behavioral/chaining_method.py) | allow calling multiple methods on the same object in a single statement |
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|:-------:| ----------- |
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| [chain_of_responsibility](patterns/behavioral/chain_of_responsibility.py) | apply a chain of successive handlers to try and process the data |
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| [catalog](patterns/behavioral/catalog.py) | general methods will call different specialized methods based on construction parameter |
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| [chaining_method](patterns/behavioral/chaining_method.py) | continue callback next object method |
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| [command](patterns/behavioral/command.py) | bundle a command and arguments to call later |
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| [interpreter](patterns/behavioral/interpreter.py) | define a grammar for a language and use it to interpret statements |
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| [iterator](patterns/behavioral/iterator.py) | provide a way to access elements of a collection sequentially |
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| [mediator](patterns/behavioral/mediator.py) | encapsulate how a set of objects interact |
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| [memento](patterns/behavioral/memento.py) | capture and restore an object's internal state |
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| [observer](patterns/behavioral/observer.py) | allow objects to notify other objects about changes in their state |
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| [publish_subscribe](patterns/behavioral/publish_subscribe.py) | allow objects to subscribe to events and receive notifications when they occur |
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| [registry](patterns/behavioral/registry.py) | keep track of all instances of a class |
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| [specification](patterns/behavioral/specification.py) | define a set of criteria that an object must meet |
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| [state](patterns/behavioral/state.py) | allow an object to change its behavior when its internal state changes |
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| [strategy](patterns/behavioral/strategy.py) | define a family of algorithms and make them interchangeable |
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| [template](patterns/behavioral/template.py) | define the skeleton of an algorithm in a method, allowing subclasses to override specific steps |
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| [visitor](patterns/behavioral/visitor.py) | separate an algorithm from the object structure it operates on |
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| [iterator](patterns/behavioral/iterator.py) | traverse a container and access the container's elements |
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| [iterator](patterns/behavioral/iterator_alt.py) (alt. impl.)| traverse a container and access the container's elements |
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| [mediator](patterns/behavioral/mediator.py) | an object that knows how to connect other objects and act as a proxy |
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| [memento](patterns/behavioral/memento.py) | generate an opaque token that can be used to go back to a previous state |
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| [observer](patterns/behavioral/observer.py) | provide a callback for notification of events/changes to data |
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| [publish_subscribe](patterns/behavioral/publish_subscribe.py) | a source syndicates events/data to 0+ registered listeners |
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| [registry](patterns/behavioral/registry.py) | keep track of all subclasses of a given class |
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| [servant](patterns/behavioral/servant.py) | provide common functionality to a group of classes without using inheritance |
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| [specification](patterns/behavioral/specification.py) | business rules can be recombined by chaining the business rules together using boolean logic |
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| [state](patterns/behavioral/state.py) | logic is organized into a discrete number of potential states and the next state that can be transitioned to |
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| [strategy](patterns/behavioral/strategy.py) | selectable operations over the same data |
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| [template](patterns/behavioral/template.py) | an object imposes a structure but takes pluggable components |
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| [visitor](patterns/behavioral/visitor.py) | invoke a callback for all items of a collection |
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## Design for Testability Patterns
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| Pattern | Description |
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|:-------:| ----------- |
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| [dependency_injection](patterns/dependency_injection.py) | 3 variants of dependency injection |
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## Fundamental Patterns
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| Pattern | Description |
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|:-------:| ----------- |
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| [delegation_pattern](patterns/fundamental/delegation_pattern.py) | an object handles a request by delegating to a second object (the delegate) |
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## Others
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| Pattern | Description |
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|:-------:| ----------- |
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| [blackboard](patterns/other/blackboard.py) | architectural model, assemble different sub-system knowledge to build a solution, AI approach - non gang of four pattern |
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| [graph_search](patterns/other/graph_search.py) | graphing algorithms - non gang of four pattern |
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| [hsm](patterns/other/hsm/hsm.py) | hierarchical state machine - non gang of four pattern |
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## Videos
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* [Design Patterns in Python by Peter Ullrich](https://www.youtube.com/watch?v=bsyjSW46TDg)
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* [Sebastian Buczyński - Why you don't need design patterns in Python?](https://www.youtube.com/watch?v=G5OeYHCJuv0)
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* [You Don't Need That!](https://www.youtube.com/watch?v=imW-trt0i9I)
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* [Pluggable Libs Through Design Patterns](https://www.youtube.com/watch?v=PfgEU3W0kyU)
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## Contributing
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When an implementation is added or modified, please review the following guidelines:
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##### Docstrings
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Add module level description in form of a docstring with links to corresponding references or other useful information.
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Add "Examples in Python ecosystem" section if you know some. It shows how patterns could be applied to real-world problems.
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[facade.py](patterns/structural/facade.py) has a good example of detailed description, but sometimes the shorter one as in [template.py](patterns/behavioral/template.py) would suffice.
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##### Python 2 compatibility
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To see Python 2 compatible versions of some patterns please check-out the [legacy](https://github.com/faif/python-patterns/tree/legacy) tag.
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##### Update README
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When everything else is done - update corresponding part of README.
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##### Travis CI
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Please run the following before submitting a patch:
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- `black .` This lints your code.
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- Either `tox` or `tox -e ci37` for unit tests.
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- If you have a bash compatible shell, use `./lint.sh`.
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## Contributing via issue triage [![Open Source Helpers](https://www.codetriage.com/faif/python-patterns/badges/users.svg)](https://www.codetriage.com/faif/python-patterns)
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You can triage issues and pull requests on [CodeTriage](https://www.codetriage.com/faif/python-patterns).

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